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Sunday, 30 October 2016

Repairing bicycle wheel bearing cones

How to repair pitted cones with simple equipment

Many, maybe most, old bikes we repair have at least one wheel bearing cone with pits in the track where the balls roll. Here’s an example with pretty bad damage:
click on the picture to see closer in
Cones aren’t always that bad, often it’s only a single small pit.
If you keep riding on damaged cones, you can wreck the whole hub. Over time the damage to the cone is likely to get worse, then balls will be damaged, and then the bearing cup in the wheel hub will be damaged in turn. Pitted cups are rarely able to be replaced, we haven’t worked out how to repair them, so this is usually the end of a wheel – or at least the hub.
It appears to us that the usual cause of damaged wheel bearings is over-tightening of the wheel bearings. This is widespread bad practice: most brand new bikes we check have seriously over-tightened wheel bearings, and often have damaged cones after only a little riding.
It’s really important to learn to grease and adjust wheel bearings, or find a bike mechanic who does it well. Don’t presume a bike shop does it well, especially if they are selling new bikes with badly adjusted bearings.
Sheldon Brown describes how to adjust bearings in this page:
Park Tools have a detailed tutorial on bearing service and adjustment here:
The usual repair for a damaged cone is to replace the cone and balls, or the whole axle assembly. This is fine if you live next door to a bike shop with a comprehensive stock of cones, but there are heaps of different sizes and shapes of cones so it is not straightforward to replace them. It is also a waste to discard a whole axle assembly because it’s missing a few milligrams of steel in a special place.
We’ve had very good results with grinding and polishing the bearing surface of damaged cones and using them again – for 1000s of km. Once or twice I’ve had re-ground cones develop a groove along the ball track, presumably because I ground off a very thin layer of case hardening, leaving only soft metal. However most cones appear to have deep enough hardness to be re-ground and work well. The job only takes a few minutes, in addition to the time it already takes to dis-assemble and inspect the bearings.
Jasper and I have developed slightly different methods. I use an angle grinder, he uses a dremel with a small grinding cylinder. Both methods have yielded good results.
We start by locking the cone and a locknut together on their axle, with about 40mm of axle behind the locknut to go into the chuck of a drill press. Like this:

If you use an angle grinder, the drill press table needs to be adjusted so the disc guard can be rested on the table, while the grinding disc contacts the cone. Then you need a steady hand. Here is how we do it:

Using a dremel (Jasper uses a cheaper “Demel” from China) is easier, perhaps a little slower, and smoother. Here’s how Jasper does it:

Here's a cone ground with the dremel, unpolished:
After grinding, the cone needs to be polished. We wrap some sandpaper around a small cylinder (such as a round pencil or a piece of brake cable outer) and sand the spinning cone until smooth.
Then the cone looks like new:

This is bush engineering, and it may appear rough. However they usually feel very good after greasing and assembling, and nearly all have lasted very well in heavy use. As far as I can see, most cheaper new wheel bearing cones aren’t ground after heat treatment, so they aren’t super accurate. The action of the balls rolling around tends to deal with small inaccuracies, and if greased and adjusted carefully, the bearing will make itself more true with time.

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